Compact disks (CDs) are storage devices for audio signals encoded as digital data. Audio CD players convert the CD digital data to analog data, which are directly reproducible as audio signals.
Conventional CDs contain digitized audio data, generally without any data reduction or compression. They are typically capable of storing 650 megabytes of data, which is roughly equivalent to 74 minutes of high-quality standard audio transmission. The conventional audio CD is relatively impervious to normal usage, user-friendly, inexpensive to manufacture, and produces high-quality sound.
The Moving Picture Experts Group (MPEG) has defined encoding standards for moving pictures and audio signals. MPEG encoding compresses signals using encoding algorithms that produce a slightly less than exact reproduction of the picture, or audio signal, but in a way that is normally largely unnoticeable to the human senses. The MPEG standards for encoding audio signals are known as MPEG Audio Layers-1, -2 and -3, with each layer being progressively more compressed than that before it. Layer-1 compresses digital audio data approximately 4-fold; Layer-2, 6- to 8-fold; and Layer-3, 10- to 12-fold. Whereas uncompressed analog-to-digital encoding requires nearly 10 megabytes of data for 1 minute of high-quality stereo sound, with MPEG Layer-3 encoding, the same 1 minute of high-quality stereo sound requires less than 1 megabyte of data.
MPEG encoding is used for visual and video encoding, for example, in DVDs (Digital Video Disks), which are viewable on MPEG-enabled DVD video players. Fraunhofer Gesellschaft IIS-A, of Erlangen, Germany, offers MPEG-encoded audio/visual CD-ROMs designed for use with a personal computer having a sufficiently powerful CPU and suitable decoding software. MPEG Layer-3 audio encoding is also available for use in computers, network-based radio, and other applications that require large audio signal data bases.
To date MPEG compression and decompression require the use of a computer. For on-line, real-time play, particularly of MPEG Layer-3, which is relatively computation-intensive, a powerful computer, based at least on an Intel Pentium MMX processor, for example, is generally needed. Consequently, MPEG audio compression has not been applied to home entertainment or portable audio devices.
European Patent Application EP 0786774 describes a digital portable stereo headphone player for recording and playback of compressed audio data on a mini-disk. The player includes a compression/expansion circuit, but no details of this circuit are described. The player is complex and appears to have many of the features of a personal computer system.
European Patent Application EP 0772192 describes optical disk playback apparatus, which is capable of receiving disks that include either compressed or non-compressed audio data. If a disk including compressed data is received, the apparatus converts the data to non-compressed data.